[tf2_bullet] Fix CMake warning

The warning was
```
--- stderr: test_tf2
CMake Warning at /workspaces/ros2/src/geometry2-1/install/tf2_bullet/share/tf2_bullet/cmake/ament_cmake_export_libraries-extras.cmake:116 (message):
  Package 'tf2_bullet' exports library 'tf2_bullet' which couldn't be found
Call Stack (most recent call first):
  /workspaces/ros2/src/geometry2-1/install/tf2_bullet/share/tf2_bullet/cmake/tf2_bulletConfig.cmake:41 (include)
  CMakeLists.txt:27 (find_package)
```

Author: gleichdick

tf2_bullet/CMakeLists.txt

@@ -46,5 +46,4 @@ if(BUILD_TESTING)
 endif()
 
 ament_export_include_directories(include)
-ament_export_libraries(${PROJECT_NAME})
 ament_package()